The present invention relates to medical electrical leads for sensing or electrical stimulation of body organs or tissues and their method of fabrication, such leads having multiple electrical conductors encased in a lead body, and particularly to implantable cardiac leads for delivering electrical stimulation to the heart, e.g., pacing pulses and cardioversion/defibrillation shocks, and/or sensing the cardiac electrogram (EGM) or other physiologic data.
Implantable medical electrical stimulation and/or sensing leads are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation, and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters of the body. A pacemaker or cardioverter/defibrillator implantable pulse generator (IPG) or a cardiac monitor is typically coupled to the heart through one or more of such endocardial leads. The proximal end of such leads typically is formed with a connector which connects to a terminal of the IPG or monitor. The lead body typically comprises one or more insulated, conductive wire surrounded by an insulating outer sleeve. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. An endocardial cardiac lead having a single stimulation and/or sensing electrode at the distal lead end and a single conductive wire is referred to as a unipolar lead. An endocardial cardiac lead having two or more stimulation and/or sensing electrodes at the distal lead end and two or more conductive wires is referred to as a bipolar lead or a multi-polar lead, respectively.
In order to implant an endocardial lead within a heart chamber, a transvenous approach is utilized wherein the lead is inserted into and passed through a pathway comprising the subclavian, jugular, or cephalic vein and through the superior vena cava into the right atrium or ventricle. It is necessary to accurately position the sense and/or stimulation electrode surface against the endocardium or within the myocardium at the desired site in order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber. The desired heart sites include the right atrium, typically the right atrial appendage, the right ventricle, typically the ventricular apex, and the coronary sinus and great vein.
The transvenous pathway can include a number of twists and turns, and the lead body can be forced against bony structures of the body that apply stress to it. Moreover, the heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses at least the distal portion of the lead body. The lead conductors and insulation are subjected to cumulative mechanical stresses, as well as material reactions as described below, that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on device performance and patient well being.
Early implantable, endocardial and epicardial, bipolar cardiac pacing leads employed separate coiled wire conductors in a side by side configuration within a silicone rubber sheath and incorporated a lumen for receiving a stiffening stylet inside the lumen of at least one of the conductor coils to facilitate advancement through the transvenous pathway. The stiffening stylet was advanced through a proximal connector pin opening to stiffen the lead body during the transvenous introduction and location of the distal electrodes deeply inserted into the right ventricular apex and was then withdrawn. The relatively large diameter and stiff lead body provided column strength that was relied upon to maintain the distal electrodes embedded into the trabeculae of the right ventricular apex. Fibrous tissue growth about the distal lead body was also relied upon to hold the distal pace/sense electrodes in position.
Similar atrial, J-shaped lead bodies were developed that relied upon the lead body stiffness and shape to lodge and maintain distal pace/sense electrodes lodged into the right atrial appendage after the stiffening stylet was removed from the lead conductor lumen. In the case of early J-shaped atrial leads formed of silicone rubber, the lead body was reinforced with an outward extending silicone rubber rib to maintain the J-shape bend when the stylet was removed. In later J-shaped atrial leads, internally encased metal coils or wires have been employed to maintain the J-shape bend.
Such relatively large and stiff lead bodies were disadvantageous in a number of respects. The available bio-compatible conductor material alloy presented an impedance that limited current carrying capacity. The large diameter body made it difficult to implant more than one lead through the venous system. The relatively high column strength was often still insufficient to maintain the pace/sense electrodes in the atrial appendage or ventricular apex, and physicians often resorted to leaving the stylets in place, resulting in fracture of the lead conductor and lead body sheath when the stylet wire broke. Once the lead bodies fibrosed in, they were difficult to retract from the heart if they needed to be replaced. Finally, the lead conductors tended to fracture at stress sites, in bipolar leads sometimes due to stresses applied unevenly to the side-by-side arrangement of the conductor coils.
In the efforts to solve these problems, more flexible lead bodies were developed using smaller diameter coiled wire conductors and other insulating materials, most notably polyurethane compositions. Passive and active fixation mechanisms incorporated were into the distal end of the endocardial lead to fix the electrode at a desired site in a heart chamber during the acute post-operative phase before fibrous tissue growth envelops the lead body. Passive fixation mechanisms, e.g., a plurality of soft, pliant tines that bear against the trabeculae in the right ventricle or the atrial appendage to urge the distal tip electrode against the endocardium, do not invade the myocardium. Active fixation mechanisms are designed to penetrate the endocardial surface and lodge in the myocardium without perforating through the epicardium or into an adjoining chamber. The most widely used active fixation mechanism employs a sharpened helix, which typically also constitutes the distal tip electrode, that is adapted to be rotated by some means from the proximal end of the lead outside the body in order to screw the helix into the myocardium and permanently fix the electrode at the desired atrial or ventricular site.
The side by side, bipolar, coiled wire lead body design was also replaced by a coaxial configuration which is more resistant to fracture and smaller in diameter and which was typically formed of polyurethane or silicone rubber inner and outer sheathes. More recently, each such coiled wire conductor of both unipolar and bipolar leads was formed of a plurality of multi-filar, parallel-wound, coiled wire conductors electrically connected in common in an electrically redundant fashion as shown in commonly assigned U.S. Pat. No. 5,007,435, for example, incorporated herein by reference. Such redundant coiled wire conductors of bipolar and multi-polar lead bodies are coaxially arranged about the stiffening stylet receiving lumen and insulated from one another by coaxially arranged insulating sheaths separating each coiled wire conductor from the adjacent coiled wire conductor(s).
In the implantation of a cardiac device of the types listed above, and in the replacement of previously implanted cardiac leads, two or more transvenous cardiac leads are typically introduced through the venous system into the right chambers or coronary sinus of the heart. It has long been desired to minimize the diameter of the transvenous cardiac lead body to facilitate the introduction of several cardiac leads by the same transvenous approach. Moreover, a number of multi-polar, endocardial cardiac leads have been designed to accommodate more than two electrodes or to make electrical connection with other components, e.g., blood pressure sensors, temperature sensors, pH sensors, or the like, in the distal portion of the lead. In addition, endocardial cardioversion/defibrillation leads were developed for unipolar or bipolar pacing and sensing functions and for delivering cardioversion/defibrillation shocks to a heart chamber intended to be implanted in a heart chamber or a cardiac blood vessel, e.g., the coronary sinus. The increased number of separate polarity and insulated coiled wire conductors is difficult to accommodate in the conventional coaxial coiled wire conductor winding arrangement having a desired, small, lead body outer diameter. One approach involved the use of separately insulated, coiled wire conductors that are parallel-wound with a common diameter and are separately coupled between a proximal connector element and to a distal electrode or terminal as disclosed in commonly assigned U.S. Pat. No. 5,796,044, incorporated herein by reference.
Moreover, the use of thin polyurethane inner and outer sheathes along with certain lead conductor alloys became problematic as the bio-stability of such lead materials in chronic implantation came into question as described in commonly assigned U.S. Pat. No. 5,419,921. In general, it is acknowledged that there are a number of mechanisms for degradation of elastomeric polyurethane pacing leads in vivo. One is environmental stress cracking (ESC), the generation of crazes or cracks in the polyurethane elastomer produced by the combined interaction of a medium capable of acting on the elastomer and a stress level above a specific threshold. Another is metal ion induced oxidation (MIO) in which polyether urethane elastomers exhibit accelerated degradation from metal ions such as cobalt ions, chromium ions, molybdenium ions and the like which are used alone or in alloys in pacing lead conductors. As explained therein, certain polyurethane elastomers that have desirable characteristics for lead bodies are more susceptible to ESC and MIO degradation than others that are less desirable. In the ""921 patent, the polyurethane elastomers that are susceptible to ESC and MIO degradation are coated or co-extruded with the less susceptible polyurethane elastomers to form tubular sheaths having their inner and outer surfaces protected by a less susceptible material layer.
All of the above considerations as to the increased complexity of the leads, the number of leads implanted in a common path, and desire to advance leads deep in the relatively small diameter coronary veins have led to efforts to at least not increase and optimally to decrease the overall diameter of the cardiac lead body without sacrificing bio-stability, resistance to crushing forces, and usability. It has been proposed to diminish the lead body further by eliminating the lumen for receiving the stiffening stylet and by replacing the large diameter coiled wire conductors with highly conductive miniaturized coiled wire conductors, stranded filament wires, or cables formed of a plurality of such stranded filament wires. In bipolar or multi-polar leads, each such wire or cable extends through a separate lumen extending in parallel within a lead body sheath that maintains electrical isolation between them.
Examples of such lead body insulating sheaths formed to enclose a plurality of straight, typically stranded, wire lead conductors, miniaturized coiled wire conductors or combinations of such straight and coiled wire conductors are disclosed in U.S. Pat. Nos. 4,608,986, 5,324,321, 5,545,203, and 5,584,873, all incorporated herein by reference. These patents and U.S. Pat. Nos. 4,640,983, 4,964,414, 5,246,014, 5,483,022, and 5,760,341, all incorporated herein by reference, present a number of alternative designs of such stranded filament wires or cables.
In the ""873 patent, a unitary lead body insulating sheath is extruded having a plurality of spaced apart, outer lead conductor lumens that extend longitudinally and in parallel to one another for receiving coiled and/or straight wire conductors extending therethrough. The sheath is extruded in a single piece of a single material, and a like plurality of compression lumens that are preferably tear drop shaped are formed and extend longitudinally between the conductor lumens that absorb compression force that otherwise would crush a solid extruded lead body sheath. In certain embodiments an inner, centrally disposed lumen is formed in the lead body sheath that can be employed as a lead conductor lumen or as a compression lumen that can be made large enough in diameter to receive a stiffening stylet during introduction of the lead.
The above-referenced U.S. patent application Ser. No. 08/990,647 discloses a lead body sheath formed of separate parts including an extruded strut member or core and a separately extruded tubular outer tube. The core is extruded to form a plurality of longitudinally extending grooves in which lead conductors may be located and the assembly of the core and lead conductors is fitted within the lumen of the outer tube which thereby encloses the core and holds the conductors in the grooves. This construction simplifies the manufacture of the lead bodies, as it allows the conductors simply to be laid in the elongated grooves of the core rather than requiring that they be pushed or pulled along the lengths of pre-formed lumens. In some embodiments, the core is provided with a central, reinforcing strand, extending along the length of the lead body, providing for structural integrity and high tensile strength. The core may be manufactured as a single extrusion, extending the entire length of the lead body, or may take the form of sequentially aligned multiple extrusions of differing materials to provide for differential stiffness along the length of the lead.
In all of these lead body designs, the adjacent conductor lumens are separated from one another by very thin webs of the extruded insulating material. Despite these improvements, the cumulative affects of applied bending stresses can cause the extruded insulation webs of the lead body to split, thereby allowing the adjacent lead conductors to contact one another and to short-circuit the electrodes they are connected with. Each conductor lumen except for a centrally disposed conductor lumen is also separated from the outer surface of the insulating body sheath by a thin web. This outer thin web can also split, exposing the conductor to body fluids and tissues. The loss of support of the lead conductors upon splitting of the lead body sheath webs can also result in excessive bending and eventual fracture of the conductor. This problem is exacerbated when lead conductors of differing types each having a differing bending stiffness are enclosed in the outer lead conductor lumens leading to a lead body that is more flexible when bent in one direction than when bent in another direction.
The present invention addresses these problems by forming a lead body comprising an electrically insulating lead body sheath enclosing one or more lead conductors and separating the lead conductors from contacting one another. The lead body sheath is co-extruded in a co-extrusion process using bio-compatible, electrically insulating, materials of differing durometers in differing axial sections thereof, resulting in a unitary lead body sheath having differing stiffness axial sections including axial segments or webs or lumen encircling rings or other structures in its cross-section. The selection of the durometer of the materials and the configuration of the co-extruded lead body sheath sections may be used to control the geometric propertiesxe2x80x94e.g. bending stiffness, torsional stiffness, axial tension-compression stiffness, shear stiffness, and transverse compression stiffness of the lead body sheath. The lead body sheath is co-extruded to have an outer surface adapted to be exposed to the environment or to be enclosed within a further outer sheath and to have a plurality of lead conductor lumens for receiving and enclosing a like plurality or a fewer number of lead conductors.
In one embodiment, the lead body is co-extruded of a plurality of sheath segments, each segment containing a lead conductor lumen and formed of a first durometer material, and of a web of a second durometer material extending between the adjoining boundaries of the sheath segments. The web bonds with the adjacent segment boundaries to form the unitary lead body insulating sheath. The web may be formed by co-extrusion of a higher durometer material than the first durometer material.
In a further embodiment, the lead body sheath is co-extruded of a plurality of sheath segments, wherein each sheath segment contains a lead conductor lumen and is formed of a selected durometer material, whereby the lead body sheath can be tailored to exhibit differing bending stiffness away from the lead body sheath axis in selected polar directions around the 360xc2x0 circumference of the sheath body.
This embodiment is particularly suitable for use with lead conductors of differing types that have differing bending stiffnesses. In one application of this embodiment, the durometer of the sheath segments are selected in relation to the lead conductor to compensate for the lead conductor bending stiffness. For example, relatively stiff lead conductors can be enclosed in segment lumens formed within segments of relatively low stiffness due to relatively low durometer materials, whereas relatively flexible lead conductors can be enclosed in segment lumens formed within segments of relatively high stiffness due to relatively high durometer materials. In this way, the bending stiffness of the lead body in all polar directions through 360xc2x0 can be brought into equilibrium.
In a further application of this embodiment, it may be desired to form a lead body that does exhibit a bias to bend more readily in one polar direction than in the other directions. In this case, one of the sheath segments can be co-extruded of a more flexible material than the other sheath segments, and can enclose a relatively flexible lead conductor within that sheath segment lumen.
In a third embodiment, the web of the first embodiment and the differing stiffness sheath segment materials of the second embodiment can be advantageously combined. In this way, additional control of cross section geometric properties may be achieved by the use of different durometer materials not only for the sheath/webs, but also for each sheath segment around the cross section of extruded lead body material containing the lumens.
The sheath segments are preferably shaped as arcuate sections of the generally circular cross-section insulating sheath, and each preferably encloses a single lead conductor lumen. In a fourth embodiment, a centrally located lead conductor and/or stiffening stylet receiving lumen can also be formed of the same or differing durometer material as the sheath segments of the first or second embodiment or surrounded by the web material of the first or third embodiment.
In these embodiments of the invention, the adjoining boundaries of the longitudinally extending sheath segments with one another or with an intervening web are in intimate physical contact and bonded with one another in the co-extrusion process.
Thus, the co-extruded lead body sheath may be constructed in a variety of ways and may or may not be enclosed in a co-extruded outer sheath. The lead body sheath may contain a plurality of lumens either empty or containing electrical conductors, with or without a co-extruded sheaths surrounding each lumen. The lead body sheath may be co-extruded in sheath segments, sheath each segment connected by a radially oriented co-extruded sheath of various geometries which may or may not contact sheaths lining the lumens.
The co-extrusion of lead body sheath allows for selective control of lead body stiffness for bending, torsion, axial tension or compression, shear and transverse compression over the full length of the lead body or for a localized portion of the lead body length. Moreover, it allows the sheath segments and/or webs to act as barriers to prevent crack propagation in the lead body sheath which might lead to electrical contact between the lead conductors producing a short or incorrect signals, or cracking to the outer surface allowing the electrical lead conductors to be damaged by exposure to body fluids.
This summary of the invention and the advantages and features thereof have been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted.